1. Field of the Invention
The present invention relates to an exposure apparatus, an exposure method, and a device manufacturing method.
2. Description of the Related Art
An exposure apparatus exposes a substrate (a wafer formed with a resist layer on the surface thereof, or the like) via a pattern of an original (a reticle or the like), in a lithography step included in a manufacturing step of a semiconductor device, a liquid crystal display device or the like. Among this apparatus, there is a step-and-scan type exposure apparatus projecting an image of the pattern of the original to each shot area (a pattern transfer area) on the substrate by scanning the original and the substrate in synchronization, in the manufacturing step of the semiconductor device. In the scanning type exposure apparatus, it is necessary to set an (accumulated) exposure amount (dose) to each point in the exposure area within an allowable range based on a predetermined exposure amount, and to minimize uneven illuminance, in a similar manner to a step-and-repeat type one shot exposure apparatus. Then, for example, the exposure apparatus using an excimer laser as an exposure light adjusts an exposure amount by adjusting an applied voltage to a laser chamber in a light source and controlling pulse energy of the laser.
FIG. 10 is a graph illustrating the pulse energy of the laser (light emission output) versus the applied voltage to the laser chamber, as a characteristic of the excimer laser. Hereinafter, the characteristic is referred to as “E-V characteristic”, or only referred to as “E-V”. The E-V characteristic changes with a time depending on the density and temperature of the gas filled into the laser chamber or a discharge electrode condition. Accordingly, it is necessary to appropriately correct the E-V characteristic when the pulse energy is controlled based on the E-V characteristic. Japanese Patent No. 2785157 and Japanese Patent No. 3259222 respectively disclose an exposure apparatus for determining the E-V characteristic from the laser pulse energy emitted during exposure and the applied voltage at that time, and for determining the applied voltage based on a newest E-V characteristic. Moreover, Japanese Patent No. 3125307 discloses an exposure apparatus that performs E-V measurement for each of a number of a predetermined unit pulse or unit time, or each time of gas exchange in the laser chamber and corrects a control parameter.
The laser light of the excimer laser includes energy variation about plus or minus 10 percent per pulse. Thus, it is desirable to irradiate a plurality of pulses to average the variation, in order to precisely measure the E-V characteristic of the excimer laser. The number of irradiation pulses required for the measurement of the E-V characteristic depends on the required measuring precision, and in general, this is equal to or more than hundreds of pulses. Further, in order to precisely measure the E-V characteristic, it is desirable to determine the E-V characteristic from the result produced by the laser pulse energy and the applied voltage to the laser chamber being measured at a plurality of times by using different set values, where light is emitted in a wide energy range.
Here, in the exposure apparatus disclosed in Japanese Patent No. 3259222, the applied voltage during exposure is determined by an exposure process, and in general, the exposure amount is controlled so as to be uniform in the shot area. Accordingly, the applied voltage is distributed over a narrow range, and it is difficult in fact to precisely measure the E-V characteristic over a wide range. Moreover, in a case where the E-V characteristic largely shifts due to events such as gas exchange of the laser, there is a possibility that the exposure cannot be correctly controlled between hundreds of the pulses until the E-V measurement and the correction process thereof is completed. In contrast, in the exposure apparatus disclosed in Japanese Patent No. 2785157, the first situation in the Japanese Patent No. 3259222 does not occur, since the applied voltage to the laser chamber during the exposure is optionally changed in sequence for the E-V measurement. However, in the method in Japanese Patent No. 2785157, the application is limited to the exposure apparatus performing the exposure, with the original and the substrate stopped. In the exposure apparatus disclosed in Japanese Patent No. 3125307, the situation in the above Japanese Patent No. 3259222 does not occur since the E-V measurement is performed each time of the predetermined event. However, a decrease of throughput may be caused since the E-V measurement is performed separately from the exposure process to the substrate.